Nonwoven fabrics, fibrillated films, and other materials comprising polymeric fibers or fibrils have been utilized in a variety of filtration and/or air-masking type applications. For example, U.S. Pat. No. 5,460,007 to Midkiff et al. discloses the use of a nonwoven web for HVAC (heating, ventilating and air-conditioning) and other air filtration media. PCT Application No. US94/12699 (Publication No. WO95/13856) discloses high-loft multicomponent fiber webs suitable for use in a variety of air filtration applications. Additionally, U.S. Pat. No. 5,855,784 to Pike et al. discloses a variety of conjugate fiber nonwoven webs suitable for use as air and/or liquid filtration media. Further, multilayer laminates have likewise been used in a variety of filtration and/or filtration-like applications, see, for example, U.S. Pat. No. 5,721,180 to Pike et al. and U.S. Pat. No. 4,041,203 to Brock et al.
Filtration materials desirably exhibit the highest filtration efficiency at the lowest possible pressure drop. In this regard, the filtration efficiencies of many filters can be improved, without a corresponding increase in pressure drop, by electrostatically charging the materials in order to impart a charge to the filter media. The use of electrets for filtration applications has been known for some time. The advantage of materials of this type is that the charge on the fibers considerably augments the filtration efficiency without making any contribution to the airflow resistance. Air filtration efficiency varies with the electrostatic charge; however, it is not a direct measure of the quantity or magnitude of charge in the media.
It is known that certain materials can be permanently electrostatically polarized, such as by heating the material, applying a high-voltage electric field, and cooling the material while under the influence of the electric field. A dielectric becomes an electret when the rate of decay of the field-induced polarization can be slowed down so much that a significant fraction of the polarization is preserved long after the polarizing field has been removed. Such electrets can be made by various methods, e.g. corona charging, triboelectric charging (friction) and so forth. Methods of treating various materials to impart an electrostatic charge are described in U.S. Pat. No. 4,215,682 to Kubic et al., U.S. Pat. No. 4,375,718 to Wadsworth et al., U.S. Pat. No. 4,588,537 to Klaase et al. and U.S. Pat. No. 5,401,446 to Tsai et al. However, the ability to impart an electrostatic charge or field of sufficient initial strength and/or maintaining a desired level of electrostatic charge over time has proven difficult for many materials and, in particular, non-polar materials such as polyolefin fabrics. Moreover, many thermoplastic polymer materials often experience a significant or accelerated degradation in the level of electrostatic charge upon exposure to heat and/or moisture. In this regard, many filtration materials are exposed to heat and/or moisture such as, for example, HVAC filtration media, sterilization wraps, vacuum bag liners, face masks and so forth.
In an effort to improve the electrostatic charge within an electret, various topical treatments have been used as a means to improve the stability of such electrostatic charges. In addition, electret nonwoven webs of non-polar polymeric materials have been provided which introduce polar groups onto side-chains and/or the backbone of the non-polar monomer or otherwise grafting unsaturated carboxylic acids thereon such as, for example, as described in U.S. Pat. No. 5,409,766 to Yuasa et al. Further, U.S. Pat. No. 4,626,263 to Inoue et al. discloses an electret treated film comprising a non-polar polymer and a non-polar polymer modified by grafting or copolymerization with a carboxylic acid, epoxy monomer or silane monomer. In addition, PCT Application US97/08482 (Publication No. WO97/44509) discloses the addition of ferroelectric particles within melt extruded thermoplastic materials in order to increase the level of electric fields imparted to the material by methods such as electric or corona polarization.
Although the above methods can provide a thermoplastic polymer material having improved levels of electrostatic charge, there still exists a need for polymeric materials having high levels of electrostatic charge. Further, there exists a need for such highly charged materials that are capable of substantially maintaining its initial charge over time. Still further, there exists a need for such a material that is capable of maintaining a substantial percent of its initial charge upon exposure to heat and/or moisture.